Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Calculator battery pack.

Status
Not open for further replies.

on1aag

New Member
I recently acquired a couple of hp 41CV's and one came with a
rechargeable battery pack. when I tried to recharge the battery pack
it didn't seem to be very cooperative and I decided to use a power
supply to give it a little push. And so the little push became a bit
bigger and bigger until my battery pack got short-circuited.
When I opened the damn thing I noticed that I got more than I expected.
And yes I fried the zener diode across the battery terminals, it was
well-done because it left it's mark on the pcb. :D
Could someone with a hp 41CV calculator measure the voltage across the
battery terminals with a fully charged pack and the charger connected.
The zener voltage could be 6V2 or 6V8, but it's also possible that they
used a custom zener. Most parts in the circuit had meaningless numbers
except for the voltage regulator, the two capacitors and the resistor.
I hope I'm not the only one who can't handle a "normal" calculator.

on1aag.
 

Attachments

  • Battery pack for hp 41CV.GIF
    Battery pack for hp 41CV.GIF
    6.5 KB · Views: 180
It looks that you have 4 cells in series which would indicate a 4.8 Volts nicad battery type.
These will get charged at max of 5.6 volts, so your zener may be something in the range of that value like 5.6, 6.2 or 6.8 Volts.
By ensuring the correct polarity and leaving the zener of while carefully charging / testing, see if you get a voltage across your battery.
 
Just have an other good look at your circuit.

The 365 ohms series resistor is there to limit the charging current from the 12 volts regulator, the zener will clamp the excess voltage to protect the battery from overcharging.

Somehow i think a spike may have killed the zener.

Have you checked the output voltage of your 12 volts regulator ?
 
Hi Rodalco,

Thanks for your reply, but I destroyed the zener diode myself.
I was trying to revive the dead batteries with a power supply
directly connected to the rechargeables. I succeeded to revive
them but fried the zener in the process. I wasn't aware that
there were other parts present in the battery pack. I would
have expected the presence of a diode and/or a thermal fuse
but not a zener diode accross the terminals of the batteries.

on1aag.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top